What will be the cutoff in UPSC 2019?

UPSC declares IES Cut off each year. The cutoffs for each exam will be released separately in a PDF format. After the declaration of Results, a merit list will be displayed online. Candidates qualifying the Prelims Cut off can then appear for Mains.
The cut-offs for last three years if we see they are between 40 to 50% of the total marks with the minimum percentage in objective papers individually. So this year also the cut off mark will be around 45 to 55%. I advise you to aim at least 60% total and at least 30% marks in two objective papers individually.

UPSC Civil Services exam is conducted in three parts - preliminary, mains and interview.

After preliminary, the cut off for appearing in mains is announced. Those who have got below that are not entitled to appear for mains. For example last year (2017) this cut off score was 105.3 for general category candidates.

These cut off vary year to year, though not significantly and depend on many factors like - total number of vacancies, total number of candidates participating, level of difficulty of questions etc.

After mains exam it's cut off is announced based on which candidates are called for interview.

The final cut off are announced after the result of interview which determines the finally selected candidates.

So practically 3 cut offs are announced time to time. The cut offs will vary from category to category also.

As a typical example the cut offs for the year 2016 for general, OBC and SC categories were -
Prelim ---- 116, 110.66, 99.34
Mains ----- 787, 745, 739
Final ------ 988, 951, 937

You can expect the future cut offs also in line with them but always be prepared for the surprises so you have to put the maximum efforts possible and forget about the cut offs. These cut offs are only a guideline, they are not the benchmarks.

UPSC civil services examination is tough in the sense that at all the stages the aspirants are screened such preliminary, main and finally the oral - interview.
Priliminary examination is to assess candidates awareness in General knowledge, General English including grammar and punctuation etc apart from the aptitude test. Mark - score will vary depending upon the category of the candidate. For the candidate of the general - rank, cut - off for the preliminary was 112 and on clearance of the said test, candidates are supposed to write the main test. Here the mark - score in some subjects will push ones ranking in the upward position because of easy scoring nature of the subjects. The scoring subjects are Philosophy, Geography, Pure Mathematics, Sanskrit etc. Hence a prudent choice of the subjects will enable a candidate to have impressive scoring. For the main examination in 2017 for the general candidates the cut - off was around 790.
Lastly, the cut - off points of each category is decided so as to be called for the final interview. Here the panel of experts would decide the overhaul performance of the aspirants and selection of the candidates would be made with the final - scoring of each category. For general candidates, the cut - off was around 990 in 2017.
This can provide a rough idea with respect to solution but one belonging to general category must aim for 1050 so that one could emerge out successful in the final test.

Your question lacks details. Every year, UPSC conducts various examinations like NDA, CDS, IES, Indian Forest Service, Geologists', IES and Indian Civil Services Examinations. You have not mentioned in which examination you are going to appear.
Further, if you are appearing in Indian Civil Services Examination (I am presuming), you have not mentioned whether you are asking about the Preliminary examination or Mains examination. I would advise you to ask question clearly giving necessary details.

So far as cut-off marks are concerned, these are finalized by the UPSC Members based upon certain criteria which are confidential. The candidates generally take the last year's cut-off marks as the base and prepare accordingly.